Optical sensors are used in a number of applications ranging from digitizing a photographic image for display or color printing to optical communications systems. Optical sensors generally operate by detecting electromagnetic energy and producing an electrical signal that corresponds to the intensity of the electromagnetic energy striking the optical sensor. Multiple optical sensors are generally used and are often geometrically positioned in arrays with individual optical sensors corresponding to a respective pixel in the resulting electronic display. The terms "pixel" and "optical sensor" as often used in the art and as used in this application are interchangeable with each other. Optical sensor arrays allow a larger spatial area to be scanned than could otherwise be performed by a single optical sensor. Some applications may use raster scan techniques in which fewer optical sensors are needed but an object or document is scanned in an incremental pattern until the object or document has been completely scanned.
Color sensitive or color discriminating optical sensors may be used to detect electromagnetic energy in specific wavelengths in the electromagnetic energy spectrum. Frequently, electromagnetic energy in discrete ranges corresponding to the colors, red, blue and green, is detected by separate color optical sensors. An electrical signal corresponding to the intensity of the electromagnetic energy from the respective optical sensors for each color is recorded to form a digitized representation of each area of the object or document. A color optical sensor array generally comprises multiple groups of photodetectors with each group sensitive to a described color and an electrical circuit corresponding to each individual photodetector in the group.
Each photodetector within a color optical sensor array produces an electrical signal in proportion to a selected range of electromagnetic energy striking the photodetector. The photodetector may have a filter or coating that allows only electromagnetic energy in the selected range of wavelengths selected to pass through and strike the photodetector. A wide variety of filters and coatings may be used. For example, photodetectors with a coating sensitive to red would only allow electromagnetic energy corresponding to the color red to strike the photodetector. The associated electrical circuit stores an electrical signal proportional to the "red" electromagnetic energy striking the photodetector.
Photodetectors are generally selected to detect electromagnetic energy in a specific bandwidth that is optimized for each application. Photodetectors can be manufactured from different materials and by different processes to detect electromagnetic energy in varying parts of the electromagnetic spectrum and over varying bandwidths within the spectrum. A photodetector can be manufactured to detect electromagnetic energy corresponding to specific parts of the electromagnetic spectrum other than just visible color. The term "color photodetector" as used in this application includes any photodetector that responds to electromagnetic energy in a specific and predetermined range of the electromagnetic spectrum. Thus, this application is not limited to photodetectors that only detect electromagnetic energy that corresponds to the colors red, green and blue.
An electrical signal from an optical sensor is typically conditioned by an output modifier. The output modifier conditions the signal or converts the electrical signal into an output signal that can be easily understood by a signal processor such as a computer. In one application, a computer may assemble the various output signals and display the resulting picture on a color monitor or print the resulting picture using a color printer. In another application, a computer may use the output signals to determine the color of water in a river to detect the amount and types of pollution. The applications in which color optical sensor arrays can be used is without bound.
Optical sensors may be manufactured in many semiconductor technologies including MOS (Metal Oxide Semiconductors), CMOS (Complementary MOS), I2L, J-FET, or Bi-CMOS. Each of the manufacturing technologies have trade offs with respect to performance, manufacturing cost, and required associated supplies and interface circuits. Optical sensors have previously been manufactured based on CCD (Charge Coupled Device) technology. Generally CCD's require a dedicated process technology, require multiple supplies, require more complicated interface electronics, and have limited capability for integrating other electronic functions and are generally more expensive than the other available technologies.
One type of color optical sensor includes a passive integrator electrical circuit. In the passive integrator, a photodiode (and its associated junction capacitance and attached parasitic capacitance) are prebiased to a high reverse voltage. The photodiode generates a photocurrent which discharges the capacitance, thereby causing the voltage to decrease. The output voltage for this type of optical sensor is generally non-linear with respect to the integrated charge since the diode capacitance is a function of the diode voltage.
A further disadvantage of the passive integrator is that the integrating capacitance (photodiode and parasitic capacitances) is determined primarily by the photodiode size. Thus sensitivity cannot be increased by increasing the photodiode size since the capacitance will increase approximately proportionally.
A further disadvantage of the passive integrator is that the high reverse voltage during operation will cause dark current to flow even in absence of light. A high dark current diminishes the usable data that can be obtained from the optical sensor. Also, variations of the dark current between photodiodes in an array due to process variation will cause an output non-uniformity.
An additional disadvantage with using some previous passive integrator electrical circuits is that in color applications, the correlation between the output signal from the photodetector varies depending upon the color, or wavelength (energy level) of the electromagnetic energy detected by the photodetector. Thus, for a given intensity of electromagnetic energy related to the color red striking the color optical sensor, the same intensity of electromagnetic energy related to the color blue will produce a lower strength electrical signal from the color optical sensor. For a given intensity of electromagnetic energy striking a photodetector, the red photodetector will produce the strongest electrical signal in comparison to green and blue photodetectors. The green photodetector produces a smaller electrical signal than the red photodetector but stronger than the blue photodetector. The blue photodetector produces the smallest electrical signal in comparison to the red and green photodetectors. Thus, the electrical signal from each color optical sensor must be normalized to correct the strength of the electrical signal to correspond to the correct representation of the color on the object or document. A prior method of normalizing the electrical signal from different color optical sensors was to program the computer processor with a correction value for each color.